1. Field of the Invention
The invention lies in the area of measuring the quality of telephone links in telecommunications systems. More in particular, it concerns measuring a talking quality of a telephone link in a telecommunication network, i.e. measuring the influence of returned signals such as echo disturbances and side tone distortions on the perceptual quality of a telephone link in a telecommunications system as subjectively observed by a talker during a telephone call.
Such a method and a corresponding device are described in the not timely published international patent application PCT/EP00/08884 (Reference [1]; for more bibliographical details relating to the references, see below under D.), which is incorporated by reference in the present application. According to the described method and device for measuring the influence of echo on the perceptual quality on the talker's side of a telephone link in a telecommunications network, a talker speech signal and a combined signal are fed to an objective measurement device, such as a PSQM system, for obtaining an output signal representing an estimated value of the perceptual talking quality. The combined signal is a signal combination of a returned signal originating from the network and corresponding to the talker speech signal, and the talker speech signal itself. The described technique has the following problem. In case the returned signal contains signal components not directly related to the voice of the talker, like noise present in the telephone system, noise derived from the background noise of the talker at the other side of the telephone connection, or noise derived from interfering signals, such signal components may have a so-called masking effect, on the echo, which then results in an increase of the subjectively perceived talking quality. Objective measurement systems such as based on the Perceptual Speech Quality Measurement (PSQM) model, recommended by the ITU-T Recommendation P.861 (see Reference [2]), or on the Perceptual Evaluation of Speech Quality (PESQ), recommended by the ITU-T Recommendation P.862 (see Reference [3]), however, will interpret noise components generally in terms of a decrease in quality. An application of an objective measurement such as PSQM in an objective measurement of the quality of speech signals received via radio links is, e.g., disclosed in Reference [4]. The mentioned problem may be tried to be solved by using noise suppression or attenuation techniques as generally known in the world of speech processing (see e.g., References [5],-,[8]) or of acoustic systems (see Reference [9]). However, these known suppression or attenuation techniques are developed for optimizing listening quality, and are not suited for the measurement and optimization of talking quality. Talking quality differs from listening quality, especially in the effect of masking noise and masking by one's own voice. Noise in general decreases listening quality but increases talking quality.